Engines such as diesel or other lean burn engines generally provide more complete filet combustion and better fuel efficiency but require higher operating pressures and temperatures compared to non-lean burn engines. With the higher pressures and temperatures, nitrogen oxides (NOx) emissions including nitric oxide (NO) and nitrogen dioxide (NO2) are typically higher as oxygen and nitrogen tend to combine more easily at a higher temperature. NOx emissions cause a number of environmental issues such as smog, acid rain, excess aqueous nutrients and so on. Thus emissions control regulations limit the amount of NOx emissions of engines and necessitate the use of reduction devices in the exhaust systems in order to reduce the NOx emissions to an acceptable level.
A Selective Catalytic Reduction (SCR) catalyst device is typically used to control the NOx emissions of engines. The catalyst converts NOx gases into nitrogen gases and water with the aid of a reduction agent. The reduction agent typically contains hydrogen or the like which is capable of removing oxygen from NOx gases. Commonly used reduction agents are ammonia, Diesel Exhaust Fluid (DEF), urea, hydrocarbon-containing compounds and the like. The reduction agent is added to the catalyst and is adsorbed onto the catalyst to facilitate the reduction process. Typically, a solution of the reduction agent is internally or externally carried by an engine, and a supplying system injects the reduction agent into the exhaust gas stream entering the catalyst.
The reduction agent is typically adsorbed into the catalyst. The amount of reduction agent stored on the catalyst is generally proportional to the amount of NOx gases that can be converted into nitrogen gases and water. Thus, to increase the NOx conversion ratio, it is desirable to increase the amount of reduction agent stored on the catalyst. When the reduction agent supplied is not fully adsorbed on the catalyst, the remaining reduction agent that is unreacted can undesirably be carried by the exhaust flow downstream of the catalyst and is released into the atmosphere (i.e., slip).
The amount of adsorption of NH3 in a SCR catalyst partially depends on a catalyst's temperature. When the catalyst operates at low temperatures, the amount of adsorption of NH3 in the catalyst is high. However, at high temperatures, the amount of adsorption of NH3 in the catalyst is reduced due to instability of the adsorbed NH3 species and thus the NH3 past the catalyst can increase. In addition to such a temperature effect, the catalyst's efficiency can degrade over time. As the catalyst ages, the storage capacity of NH3 in the catalyst can decrease, thereby resulting in an increase in the amount of NH3 slip. Alternatively, as the catalyst ages, the storage capacity of NH3 in the catalyst can increase, thereby resulting in a decrease in the amount of NH3 slip. Thus, the efficiency of the catalyst and the amount of NH3 slip past the catalyst depends, in part, on the service time and temperature of the catalyst.
One way to minimize the impact of the aging and to avoid NH3 slip in a SCR catalyst is to optimize operating conditions of the catalyst. In this approach, a SCR catalyst is operated for a predetermined period of time to determine a stable NOx reduction ratio, NH3 storage, and NH3 slip of the catalyst. Based on the determined stable NOx reduction ratio, NH3 storage, and NH3 slip, the operating conditions of the catalyst are determined. However, this approach has some drawbacks because an engine does not always operate under the same conditions over the life of the catalyst.
An alternative method of controlling an SCR process is described in U.S. Pat. No. 8,156,729 (the '729 patent) issued to Sun on Apr. 17, 2012. The '729 patent discloses steps for determining the age of the SCR with an age factor; determining SCR efficiency by using SCR age as a factor with an age factor; determining desired engine out NOx emission by using current SCR efficiency and desired tailpipe NOx emissions; managing engine control setpoints which form a forward control portion of the ECS by using fuzzy logic; and controlling engine out NOx emissions by using closed loop engine out NOx sensor feedback. However, this approach does not determine the necessary amount of a reduction agent to maintain the desired engine out NOx emission. The desired engine out NOx emission changes as the catalyst ages. However, without determining the necessary amount of the reduction agent to cope with the changing desired NOx reduction ratio over time, unnecessarily large amounts of a reduction agent can be supplied to the aging catalyst, thereby causing excess NH3 slip past the catalyst.
Accordingly, there is a need for a system which efficiently utilizes a SCR device, achieves low NOx emissions, minimizes NH3 slip past the catalyst and/or increases fuel efficiency for an engine based on more accurate aging information.